Methods of this kind are applied, for example, in measuring devices, or field devices, of automation technology and/or process measurements technology, for ascertaining process variables, such as e.g. interface-level, fill-level or separating-layer-level, of at least one medium in a process. Manufactured and sold by the assignee are, for example, measuring devices under the designations Micropilot, Ultrasonic, Levelflex and Multicap, whose primary task is to ascertain and/or monitor fill-level of a medium in a container. In one of a large number of travel-time measuring methods, for example, using guided microwave, time-domain reflectometry, or the TDR-measuring method (Time Domain Reflection), a high-frequency pulse is transmitted along a Sommerfeld or Goubau waveguide, or coaxial waveguide, then subsequently to be partially reflected back at a discontinuity of DK-value (dielectric constant) of a medium surrounding the waveguide. From the time difference between the emitting of the high-frequency pulse and receipt of the echo signal reflected from the medium, fill-level can be ascertained. The FMCW-method (Frequency Modulated Continuous Waves), in which the frequency region of a continuous measurement signal is altered and distance determined from the frequency difference of the transmitted measurement signal relative to the reflected measurement signal, can likewise be performed in connection with the above principle of measurement.
In all known physical measuring principles, most often, a compromise must be made as regards accuracy and reliability of measurement. Thus, based on surrounding conditions and properties of the medium in the particular application, that physical principle of measurement is selected, whose advantages best outweigh its disadvantages. Fundamentally, the freely radiating measuring methods of process measurements technology employ a number of physical principles of measurement, among these being microwave travel-time measurement, ultrasonic travel-time measurement and gamma-ray absorption measurement, as well as, sometimes, also laser-light travel-time measurement. And, the medium-contacting measuring methods of process measurements technology include, among others, plumb-line measuring, capacitive measurement, conductivity measurement and measurement utilizing guided microwaves. All these measuring methods have, as a function of principle of measurement, measured medium, measuring situation, process conditions and measuring performance, have their own particular sets of advantages relative to one another.
Direct comparison of the various physical measuring principles shows that selection of the measurement principle suited for a particular application is, in most cases, very difficult. However, as regards measuring level of a separating layer, already a large number of applications can be handled by the highly developed, medium-contacting measuring devices, or combinations of these measuring devices.
Time-domain reflectometry is a medium-contacting fill-level measuring method, wherein a measuring probe lies directly in contact with the medium being measured. The measuring probe is usually secured in the container via a process connection, opening or nozzle, in such a manner that the associated measuring electronics remains outside the process, i.e. not in contact with the medium. The measuring probe, in contrast, is integrated in the process.
The following references discuss, in greater detail, the measuring of level of a separating layer by means of time domain reflectometry.
EP 1 804 038 A1 discloses a method of ascertaining fill-level of a first medium, e.g. oil, in a container and for identifying presence of a second medium, e.g. water, below the first medium. In this case, microwaves are propagated on a medium-contacting waveguide and, due to discontinuities of wave resistance at disturbance locations or at media interfaces, reflected fractions of the microwaves are received back in the form of echos. A basic idea of this method involves the fact that the probe-end signal from the end of the waveguide has a sign opposite to that of echos from media interfaces. If, now, the level of the second medium reaches the end of the waveguide, the probe-end signal with opposite sign becomes covered by the echo signal of the interface between the first and second medium. So, a sign change of the probe-end signal is detected, when the interface between the first medium, oil, and the second medium, water, reaches the waveguide end.
DE 100 51 151 A1 describes a method and apparatus for detecting an upper interface of an upper liquid and a separating-layer between the upper and a lower liquid. With this method and apparatus, the wanted echos of the interface and the separating-layer, as well as the probe-end echo, are ascertained on the basis of comparison of the amplitudes of the echos with predefined, threshold values, such as e.g. start-threshold, and end-threshold.
Corresponding to the above-presented state of the art, there are various approaches for ascertaining the exact position of the wanted echo signal of the fill-level in the obtained echo curve or in the digitized, envelope curve. Dependent on the exact determining of the measuring position of the fill-level in the echo curve is, however, the accuracy of measurement that can be reached with this echo measuring principle under the given measuring conditions. The above-described methods work, per se, in each case, without problem for a large number of applications. Problems arise, however, always, when the echos of the interface and/or of the separating-layer cannot be unequivocally identified on the basis of the method.